A plurality of grooves known as score lines are formed on a face of a golf club head. The score lines are used to apply spin to a ball when the ball is struck by the face during a shot and to stabilize the amount of spin, and therefore the shape and so on of the score lines affect an striking performance and a so-called spin performance. In the case of an iron in particular, an iron having a small loft angle (between 20 degrees and 30 degrees, for example) is referred to as a long iron while an iron having a large loft angle (between 40 degrees and 60 degrees, for example) is referred to as a short iron. Numbers and symbols such as 1, 2, 3, . . . , 9, PW (pitching wedge), and SW (sand wedge) are typically allocated to irons from a long iron to a short iron.
As regards short irons known as wedges having a loft angle between 45 degrees and 60 degrees, such as a PW (a pitching wedge), an AW (approach wedge) and a SW (sand wedge), from among the irons described above, those exhibiting a superior spin performance are preferred by professional players, advanced players and so on. The reason for this is that as the loft angle increases, a flying distance of the ball decreases, and therefore the player can aim for a target destination of the ball more easily with an iron having a large loft angle. In this case, though a target area becomes narrower, it is nevertheless more advantageous for a struck ball to stop close to its landing spot rather than to run excessively.
Further, in a professional game, greens are kept hard and set such that the ball rolls over them quickly and it is therefore particularly important to apply spin to the ball so that the ball stops close to its landing spot. Likewise with regard to long irons and middle irons, those with a high spin performance generate a stable amount of spin and are therefore desirable in terms of the flying distance. This is particularly important during shots from the rough to prevent flyers, which are balls that fly further than an intended flying distance because no spin is applied thereto.
Hence, a stable spin performance is required during iron shots made under various conditions. The shape of the score line greatly affects the stability of the spin performance. Specifically, sharpening an edge portion of the score line is effective in stabilizing the spin performance. Various methods for forming this type of score line have been proposed. For example, a manufacturing method, in which an edge portion of a score line is provided on a face surface by forming the score line on the face surface through pressing and then planing the face surface evenly through planar milling, has been proposed (see Japanese Patent Application Laid-open No. 2003-199851).
Further, a method for manufacturing a high quality golf club head with excellent manufacturing efficiency by modifying the shape of a score line groove to form a trapezoidal projection and then forging the projection has been proposed (see Japanese Patent Application Laid-open No. 2003-93560). In another known technique, explosion welding is applied to a site of the face surface relating to the score lines to form a component made of a hard, wear-resistant material, whereupon score line grooves are formed on the surface by machining. This document discloses a technique of forming an R portion of the groove by machining. A dimension of the R portion is preferably smaller than 0.01 inches (0.254 mm) (see Japanese Patent Application Laid-open No. 2008-6296).
The shape of the score line is subject to rules and, according to the R&A (Royal & Ancient Golf Club of St. Andrews; hereafter referred to as R&A) and the USGA (United States Golf Association; hereafter referred to as USGA), rules are defined as to the groove constituting the score line such that a rounded edge thereof must take a circular shape having a radius of no more than 0.020 inches (0.508 millimeters), the width thereof, when measured by a 30-degree measurement method (test on file with R&A), must be no more than 0.035 inches (0.9 millimeters), an interval between sides of adjacent grooves must be no less than three times the groove width and no less than 0.075 inches (1.905 millimeters), a depth thereof must be no more than 0.020 inches (0.508 millimeters) and so on.
Furthermore, according to new rules applied by the R&A and the USGA from Jan. 1, 2010 onward, the rounded edge of the score line groove on an iron golf club having a loft angle of 25 degrees or more must conform to a two circle rule, according to which the rounded edge of the score line groove is such that its configuration in cross section is within a range between a circle that is in contact with the face surface and the side surface of the groove with a radius of 0.010 inches (0.254 millimeters) and a circle with a radius of no more than 0.011 inches (0.2794 millimeters) and concentric with the first circle.
According to these rules, the score line is subject to the following restrictions: 1. The groove width must be no more than 0.9 millimeters (0.035 inches); 2. The interval between the sides of adjacent grooves must be no less than three times the groove width; 3. The interval between the sides of adjacent grooves must be no less than 1.9 millimeters (0.075 inches); 4. The groove width must be constant; and so on.
Thus, these new standards make conventional shapes and manufacturing methods completely obsolete, hence it is therefore necessary to provide a new manufacturing method that conforms to the shape according to the new standards. However, it is difficult to manufacture such a golf club head effectively and with stability using conventional techniques such as cutting or pressing. For example, cutting processing may be implemented using a cutting tool having a corner R that conforms to the new standards, but this tool becomes worn extremely quickly, leading to an increase in cost. Furthermore, it is difficult to create a stable shape with pressing. More specifically, when a conventional processing method is employed, an edge portion may sag excessively or protrude by approximately 0.01 to 0.15 mm on a face surface side, which must be corrected.
As another problem, there is concern that, because of the new rules, the spin performance may deteriorate when professional players, advanced players and so on make shots in a manner as they played in the past. Hence, such an iron golf club is required that maintains a superior spin performance within the scope of the new rules. The score lines, in addition to being limited in shape, are provided on the striking surface and therefore, when shots are made repeatedly, the edge portions become worn, causing the spin performance to deteriorate. Hence, resistance to wear caused by impacts is also required. Methods for manufacturing a club head using a hard material exist, but with such methods feeling in striking becomes hard, making it difficult to adjust the loft angle and a lie angle.